I uploaded the atmega 8 bootloader using usbaspthen I load program using usbasp toobefore that , once after burnt the bootloader,when I press reset button, the led at pin 13 blink a little whilebut once loaded program using usbasp programmer, it seems like the bootloader gone

because when I press the reset ,the led at pin 13 wont blink

And when I change to USB, I cant upload the program because the bootloader is gone What problem is that ?

once loaded program using usbasp programmer, it seems like the bootloader gone

Yes, if you load a sketch with a programmer it will erase the bootloader. Your sketch gets control immediately on power up or reset. If you then want to be able to upload sketches via USB you have to re-burn the bootloader.

Yes, load the bootloader and then load programs using the Serial or USB connection. If you load hex files using avrdude and usbasp then the bootloader is overwritten. Decide how you want to load programs. Using serial/USB using the bootloader or the usbasp and no bootloader.